The optional field suggestion was to be able to enter a reference value onto a cash entry. There is no flow-through from other transactions - because a single cash entry could pay multiple invoices and which reference would be used for the cash?

What do you mean by the optional field 'flowing' to the recurring invoice batches? Certainly anything entered onto a cash payment transaction will not appear on the actual invoice transaction being paid, but statements can continue to list invoices with the associated payments using the 'With Matching' detail section type.

Is what you are trying to do is print a payment detail on a customer statement and have the reference from the paid invoice print? Or something else? Perhaps indicating what the desired result would be could help in giving the best solution possible.